The ISO value: Films and CCD sensors

The ISO value: Films and CCD sensors

This is the 4th part of a six-part tutorial about photography basics. So far we’ve seen

• Some physics background related to photography
• The pinhole camera and the basics of camera building
• The lenses: Zoom, Focus and Aperture

We have seen how it is possible to project an image on a surface, but we haven’t seen so far how to transform the projected image, into a photography that can be seen further in time.

In this post, we will see basics of how films and digital sensors work and how to use ISO value on your own advantage.

A little bit of Film History and the Digital Sensors

Originally photography was brought to existence because some extremely brilliant people were able to use the knowledge of some chemical substances that react to light and become other substances.

When exposed to light, these substances form sliver cristals which are opaque and dark, hence enabling people to capture black and white images based on whether the light has or hasn’t hit the surface on a particular point.

One of these cases is the family of the silver halides. That is, compounds made of silver and a halogen, usually chlorine, bromine and iodine.

Many years have passed and countless improvements over this led to very high quality photography material and further, enabled people to think on electronic, not chemical, devices capable to do the same, that is, capture images and represent them as a matrix of points, instead of an array of silver cristals.

And digital photography was born.

As you see, film photography and digital photography walk hand in hand for a very long way, despite the internals are completely different, the way they behave is very similar in many aspects, that is why the basics for film photography are very much the same than the basics for digital photography.

Sensitivity and ISO values

Two pictures of the same scene, taken with ISO200 and ISO1600 respectively. The image taken with ISO 1600 has visible noise.

In film, there is something called sensitivity of film. The more grains of silver halides in the film, the more sharp the image, but also, the more light you need. That is why film sensitivity is also commonly nicknamed film speed.

Film speeds where classified by agencies like ASA and DIN in standards like ASA 100, ASA 400 or DIN 27. Then ISO took standards over and films where named ISO 100 or ISO 400.

These standards describe the amount of light needed to take a picture with a film, the lower the ISO value, the more light needed and, usually, the higher the quality.

There are many other factors that influence the speed and quality of film, but roughly speaking, the amount of silver halides is the main reason. The amount and size of grains are equivalent to what is known in digital photography as resolution.

Digital photography has adopted the ISO film standards to describe sensor speeds and the trade-offs are about the same: the higher the ISO, the less light you need, but instead the lower the quality, which means that the image definition, contours and details, is poorer.

In digital photography, the ISO doesn’t affect the amount of pixels of the final image, hence the resolution. Instead, affects the quality of each individual pixel.

The higher the ISO value set on a digital camera, the less light you need, but also, the harder it gets to detect an image through the incoming light, producing noise on the picture.

The noise produced by a high ISO digital photography is similar, but not the same than low quality of high ISO film photography, which instead of noise is called granulation.

Anyway, roughly speaking, the trade-off with digital ISO is the same as in film ISO, you need more light to get more quality.